This invention relates to non-terrestrial digital cellular communications and, in particular, to a cellular communications network that mitigates the Doppler effects of aircraft velocity on the modulated carrier of the non-terrestrial cellular communications signals.
It is a problem in the field of digital cellular communications to provide service to aircraft-located mobile subscriber stations, due to the Doppler effects of aircraft velocity on the modulated carrier of the digital cellular signals. Digital cellular communications systems were not designed to tolerate the velocities at which many of the aircraft-located mobile subscriber stations travel and therefore these systems cannot reliably carry calls at these aircraft velocities.
Non-terrestrial cellular communications systems use existing terrestrial Code Division Multiple Access (CDMA) networks, that use orthogonal codes to implement communication channels, or Time Division Multiple Access (TDMA) networks, that use time division multiplexing of a frequency to implement communication channels, as the underlying structure to serve mobile subscriber stations that are located aboard aircraft to reduce network infrastructure costs and to enhance network capacity utilization. These digital cellular network architectures have enhanced system capacity and inherently offer many value-added services, such as Internet browsing and consumer push data applications (sports scores for instance). When the services provided by these systems are extended to non-terrestrial applications, the available market for the service provider is expanded from only the terrestrial subscriber to ubiquitous use anywhere, anytime.
The non-terrestrial digital cellular communications network is therefore a virtual overlay in three dimensional space over the CDMA or TDMA terrestrial cellular network and re-utilizes the base station controllers and mobile switches of the terrestrial CDMA or TDMA cellular network on a partitioned, or virtual basis. When the mobile subscriber stations are airborne, the wireless or xe2x80x9cairxe2x80x9d interface is accomplished by co-locating airborne-specifically configured equipment at an existing terrestrial cell site (the airborne cell site could also be a stand-alone) with specially designed, upward looking antennas. Terrestrially, these cell sites are presently operating at a carrier frequency of either 800 MHZ or 1.9 GHz and typically have cell radii of less than 40 miles. However, when configured for non-terrestrial operation with upward looking antennas, the operating range of these cell sites may approach a radius of 200 miles, with 80-100 miles being a typical operating range.
The non-terrestrial digital cellular communications network experiences technical design issues not envisioned by the designers of the terrestrial digital cellular communications network. In particular, when the aircraft travels at a high velocity, the movement of the mobile subscriber station, located in the aircraft, creates a Doppler shift or frequency shift of the modulated carrier from the observer""s perspective. In addition, when the digital cellular signals are transformed from the frequency domain to the time domain, the digital waveform and its all-critical timing can be severely and negatively impacted. The Doppler frequency shift is frequency dependent, in that the Doppler shift at 1.9 GHz is over two times the Doppler shift at 800 MHZ. Thus, there is a maximum aircraft velocity at which the non-terrestrial network can reliably carry calls for the mobile subscriber station. For some networks, such as versions of TDMA used in 800 MHZ (SMR) applications, the maximum operating velocities for aircraft based mobile subscriber stations are on the order of 130 MPH. For 1.9 GHz Personal Communication System (PCS) CDMA networks, the maximum velocities for aircraft based mobile subscriber stations approach 500 MPH (Frame Error Rates (FER) tend to become very high around 450 MPH). In either example, these maximum velocities are well below those of high-end business aircraft, commercial aircraft and military aircraft. This velocity constraint presents a serious limitation to the applicability of re-using an existing terrestrial digital cellular communications network for a multitude of airborne applications.
The above-described problems are solved and a technical advance achieved by the present Doppler insensitive non-terrestrial digital cellular communications network which ensures that, independent of the aircraft direction and apparent velocity of the mobile subscriber station, at least one and very likely two cells/antennas, carry the call even though other cells/antennas in the non-terrestrial digital cellular communications network encounter an apparent velocity of the mobile subscriber station which disables system operation.
The traditional CDMA cellular network architecture was designed to carry a wireless call between a mobile subscriber station and a base station, by simultaneously using multiple base stations or antennas to mitigate the effects of signal fading of various types, including, but not limited to: Raleigh, rician and log-normal. If one cell or one antenna in the CDMA cellular network has a poor signal for a given time frame, another cell or antenna in the CDMA cellular network which had an acceptable signal carries the call. This call management process is called soft or softer hand-off, depending on whether the call is carried between two cells or two antennas at a given cell, respectively.
For the non-terrestrial digital cellular communications network, a similar approach is taken, with the primary purpose of the call hand-off being to mitigate the Doppler effects of aircraft velocity on the modulated carrier of the digital cellular signals. Given a network topology of contiguous cells, the non-terrestrial cellular network can be designed to automatically self-compensate for the Doppler effects of aircraft velocity by having, at any given moment, at least one cell site with respect to which the aircraft has a relative velocity that is less than the system maximum. In this manner, the non-terrestrial network, not the hardware or software in the mobile subscriber station or base station, is responsible for compensating for the Doppler effects of aircraft velocity. This allows an existing CDMA terrestrial cellular network, which was not designed for the high velocities of the non-terrestrial mobile subscriber stations, to be re-used for non-terrestrial applications. Thus, with the correct network architecture and topology, a velocity constrained cellular communications network can now be simultaneously used for both terrestrial and non-terrestrial applications.
This velocity independence is achieved by having substantially complete cell extent overlap with adjacent cells. This ensures that, independent of the aircraft direction and apparent velocity of the mobile subscriber station, at least one and very likely two cells/antennas, can carry the call even though other cells/antennas in the non-terrestrial digital cellular communications network encounter an apparent velocity of the mobile subscriber station which disables system operation. Since the architecture of the non-terrestrial digital cellular communications network and the non-terrestrial communication application have a common element, namely altitude, it is also possible to minimize the Doppler/capacity problem by segmenting the non-terrestrial space into layers (also termed xe2x80x9ccell elevation sectorsxe2x80x9d), or PN code words. This topology makes use of spatial diversity in the elevation plane, or xe2x80x9cZ direction,xe2x80x9d to ensure that at least one and very likely two layers of a cell, can carry the call. By having the uppermost cell elevation sector configured for a higher level of soft and/or softer hand-off, some call handling capacity is traded for optimum system management of velocity induced Doppler frequency shift. The lower cell elevation sector(s), where aircraft fly slower is first optimized for call handling capacity. The minimum segmentation of the non-terrestrial space in a particular cell is two cell elevation sector, with the maximum segmentation of the non-terrestrial space being limited by practical implementation issues. Each cell elevation sector in the cell is formed by an antenna beam, having different system configurations to pre-set the level of soft hand-off and/or softer hand-off. In addition to shaping and directional control of antenna beams, adjustment of the forward link power and reverse link sensitivity are methods of implementing diversity in the network.
Spatial diversity within the cell site can also be implemented in the xe2x80x9cX and Y directionsxe2x80x9d, or azimuthally. This is accomplished by interleaving cells of different diameters and/or sector size and orientation. In this manner, the periodicity of the cells is randomized, keeping calls in soft hand-off or softer hand-off by maintaining a tangential direction of flight with respect to most cell sites, thereby minimizing the Doppler shift of the carrier frequency with respect to the serving cell site. This method can be employed to allow hand-offs between the non-terrestrial network and the existing terrestrial network to maximize the reuse of the network. Interleaving cells of varying size also allows for network optimization. Certain flight corridors that require additional call carrying capacity can be optimized by installing a larger number of small diameter cells, or by reorienting existing cell sites to improve the network hand-off characteristics.
The above-described attributes of the Doppler insensitive non-terrestrial digital cellular communications network allows optimization of the call handling capacity and allows compatibility with the existing terrestrial cell sites. Another benefit of this Doppler insensitive non-terrestrial digital cellular communications network is the ability to deploy the network over terrain of varying features.